How to try and catch assembly not found

OK, say I have an application like this:

using System;
using AliensExist; // some DLL which can't be found...

What I want is that if the assembly DLL AlienExist can't be found the application won't return an error - but rather be "trycatched" that is something like...:

using System;
try{
using AliensExist; // some DLL which can't be found...
} catch {}

How to do it? I know the using keyword can't be used later...but i am way too lazy now to test it.

10x!